


learning curve

by armethaumaturgy



Series: Reqs [4]
Category: Undertale (Video Game)
Genre: Alternate Universe - The Little Mermaid Fusion, Cross (X-tale) - Freeform, Dream (Dreamtale) - Freeform, Explicit Sexual Content, Foursome - M/M/M/M, Killer (Killertale) - Freeform, KillerCreamMare - Freeform, M/M, Nightmare (Dreamtale) - Freeform, Non-Human Genitalia, kcrm, sku's leviathan AU
Language: English
Status: Completed
Published: 2021-02-21
Updated: 2021-02-21
Packaged: 2021-03-19 00:01:49
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 1,254
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29617497
Author URL: https://archiveofourown.org/users/armethaumaturgy/pseuds/armethaumaturgy
Summary: “Just like that,” Dream cooed, watching Cross from over Nightmare’s shoulder.The guidance was redundant, considering Killer had his claws wrapped around Cross’ wrist, all but moving his hand himself. Cross was inwardly glad for it, really; he kind of needed it. Nightmare’s smaller form always caught him off guard, and he’d be lying if he said he’d ever pleased a cephalopod.
Relationships: Sans/Sans (Undertale)
Series: Reqs [4]
Series URL: https://archiveofourown.org/series/2151672
Comments: 4
Kudos: 177





	learning curve

**Author's Note:**

  * For [Cypress_Leaves](https://archiveofourown.org/users/Cypress_Leaves/gifts).



> this was written for @cypress_leaves on twitter ♥  
> leviathan au belongs to @skumhuu ♥

“Just like that,” Dream cooed, watching Cross from over Nightmare’s shoulder.

The guidance was redundant, considering Killer had his claws wrapped around Cross’ wrist, all but moving his hand himself. Cross was inwardly glad for it, really; he kind of needed it. Nightmare’s smaller form always caught him off guard, and he’d be lying if he said he’d ever pleased a cephalopod. The entrance he was three claws in didn’t seem all that different from his own pouch, sans the cocks, but Killer had mentioned  _ teeth _ at one point. He didn’t know if it’d been a joke or not, but it spiked his anxiety nonetheless. There were no teeth, though, as far as Cross knew.

“He’s really into it,” Killer commented. Cross would’ve almost forgotten he was right behind him, if not for the hand on his and the other shark’s tail wound around his own. It was almost too intimate, the way Cross could feel his scales rubbing against his own.

Nightmare choked out a moan, accompanied by a slew of bubbles from his mouth, as if to visualize the sound. His eyesocket opened to glare over Cross’ shoulder, where Killer was no doubt grinning. “Congratulations, you’re not blind,” he bit out, even as Dream ran his hands over his hips, rubbing soft circles into them.

“Don’t mind the commentary,” Dream said, “Keep it up, you’re doing great.”

Cross did, or at least tried to; Killer’s claws around his tightened and pushed them deeper into Nightmare. It pulled out a loud whine from the depths of his ribcage, and Cross found his skull heating up more than it already was. He was used to Nightmare’s voice being downright booming, echo-y and reverberating in his skull when he heard it. But now it was clearer, and it was easier to hear the octaves change when Killer twisted his fingers and they hit a spot that Nightmare liked.

Cross’ cocks strained against his pouch, tips peeking out just enough to have their viscous slick dripping down his abdomen.

A couple more thrusts, deep and fast, had Nightmare clamping down on the fingers, coming in a stream of off-white fluid. the tentacle wound around Cross’ forearm squeezed, and so must have the ones wrapped similarly around Killer and Dream.

Nightmare let out a keen that would forever plague Cross’ fantasies, and he pulled his fingers out despite Killer’s attempts at making them move again.

Dream cooed at his brother with soft chirps as he came back from his high, panting as his gills worked overtime to match his hammering SOUL.

It took a moment for Nightmare to compose himself, but Cross barely noticed the time passing them by, too enraptured watching the other one, and marvelling the stark difference between him and Dream, whose bright tail was lashing behind him.

Finally, Nightmare calmed down enough and unwound his tentacle from Cross’ arms. Already, he missed the way it felt gripping between his radius and ulna.

Nightmare huffed and proceeded to stare Killer down over Cross’ shoulder once more. “If I didn’t want Cross’ cocks as much as I do, I’d make sure to show you how  _ enjoyment  _ looks like.”

From behind him, Dream chuckled, sound always reminding Cross of chiming bells, and the koi moved, circling them all with a simple flick of his tail. Each time he displayed its strength and dexterity, Cross found himself more and more enamored. These were ancient, powerful beings that chose  _ him  _ for their shiver.  _ Him, _ of all monsters. It still left Cross reeling sometimes.

“I think I can manage to do that for you,” Dream said. His smile was sweet when Cross twisted his head to look at him, but the way he set his hands onto Killer’s ribcage was anything but. Another show of his hidden strength had the shark flipped over, belly up as Cross watched with wide sockets.

Killer let out a trill, unable to move where he floated now, left at Dream’s mercy. That chiming laughter was back, and Nightmare’s soon joined it. A peek at the kraken revealed his eyelight glinting with glee, glued to the way Dream ran nimble fingers over the length of Killer’s tail and slit alike. It took so little to coax Killer’s cocks out, Dream’s hand covered in their slick in no time at all.

“What a splendid idea, dear Dream,” Nightmare said, and that had been Cross’ only warning before the tentacles wound around him again tightened and yanked. A stream of bubbles left him, along with a gasp, as he was maneuvered to the other’s desire. He cursed himself for forgetting just how sneaky Nightmare could be in this form; he hadn’t noticed a single one of the tentacles ensnaring him in their hold, despite them being all over his body, wound around his tail, through his ribs, over his arms, and one even on his neck, snaking around the sensitive spaces between the vertebrae like an eel.

He found himself in the same position as Killer, body going lax despite his mind panicking for a moment, unable to move a bone. The first few moments were always the most intense ones, as he fought two instincts at once, one half of him yearning to give into the content feeling overtaking him, and the other flaring with alarm, unable to see any potential threats coming.

Nightmare’s grinning face filled his vision, the kraken clambering onto him with grace that should not have been possible with ten limbs.

Cross might not have been able to see anything but him, but he could certainly  _ hear  _ everything. Killer let out a loud moan, followed closely by Dream’s, like an echo. He could feel the water rushing past as Dream moved, soft sighs and moans that accompanied each of his tail’s swishes. He found himself wishing he could see the two of them; he wanted to know what Dream looked like. Did he take both of Killer’s cocks at once? Did Killer’s eyelight appear at the pleasure?

Nightmare snapped him out of his musings with a hand on the lower of his cocks. He hadn’t even realized they had fully emerged. Like this, Nightmare’s hand could barely encircle its girth, and it made Cross’ breath hitch when he looked down.

Nightmare moved fluidly, like the water itself, and his tentacles hid everything from view as he positioned himself. Cross couldn’t see, yet again, but he could  _ feel. _ With a satisfied sigh, Nightmare sunk onto both of the lengths at once, his entrance stretching taut around them. It was hot, and so tight that Cross jerked, body unmoving even as his joints locked up.

Embarrassingly, his first hemipene came, only further wetting the tight passage as Nightmare clenched down on him.

Killer cried out behind him, long and loud. Nightmare smirked, that much Cross could see even with his vision as blurry as it was.

“Good,” the kraken all but purred, tentacles squeezing around Cross as he started moving, not giving him a second to calm down. The suckers on his tentacles seemed to have an innate ability to latch onto the most sensitive spots that not even Cross himself knew about — who knew the inside of his ribcage could make him feel so good?

Cross could only shudder and moan as Nightmare took everything he wanted from him, by all accounts following Dream’s example.

“You’re just getting started,” he said, leaning down and running his teeth along the side of Cross’ neck not covered by a stroking tentacle. “Both of you.”


End file.
